AbstractExperiments were conducted on two Ultisols in the interior of Puerto Rico to determine the fertilizer N requirements of a high yielding, high root protein variety of cassava (Manihot esculenta Crantz cv. Llanera) from Colombia. This variety appeared to be daylength sensitive and top growth ceased from mid November to mid-February, when days were shorter than 11 1/2 hours. At Corozal (Humatas clay), the crop was grown from mid- March to mid-December and top growth responded strongly and root growth moderately to applied N. High N rates produced low root/top ratios (.67-87) but, due to the experience at the Cidra site, it is assumed that root/top ratios would have increased to higher levels had the crop continued to grow. Apparent recovery of fertilizer N was high (68-69 percent) for the 40 and 80 kg/ha rates. At Cidra (Torres clay), where the crop grew from mid-June until the following May and was dormant for the three winter months, there was no response to N, the top yields were lower but the root/top ratios were higher (average, 1.37) and were not affected by N rates. The lack of response to N was due to the high N supplying power of this soil. The maximum root yields were 23 metric tons/ha at Corozal and 33 metric tons/ha at Cidra. Plants in the highest yielding treatments contained 125-175 kg/ha N. Assuming all N was protein N, unpeeled root protein contents averaged 3.4 percent and peeled 2.3 percent on a dry weight basis and were increased only slightly by higher N rates. The N content of the 4th and 5th fully expanded leaves 4-5 months after planting was very well correlated with final root yield at Corozal (R2 = .99 for 0-160 kg/ha N rates). It appears that a minimum of 5 percent N in these leaves 2 1/2 months after planting is necessary for maximum yields.
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